1. Field of the Invention
This disclosure relates to electric motors and more specifically to electrical motor drive methods and apparatuses.
2. Description of the Related Art
The use of electric vehicles (EVs) has been promoted in recent years as an effective way to reduce crude oil consumption and emissions of harmful pollutants and green-house gasses. EVs broadly include battery powered electric vehicles (BEVs), fuel cell powered vehicles, and hybrid electric vehicles (HEVs).
HEVs employ an energy storage device such as a battery and an electrical motor drive system that are designed to optimize the energy conversion efficiency of an internal combustion engine. They also capture a portion of the kinetic energy of the vehicle for later use through dynamic braking by the motor during deceleration. An electrical motor drive system may include one or more drive units and each drive unit typically consists of a power inverter and a motor. The inverter either functions to convert a direct-current (DC) voltage to an alternating-current (AC) voltage suitable to operate the motor, or functions as a converter when the motor is operating in power generation mode. Multiple electrical drive units can be used to provide dual-wheel or four-wheel drive capabilities.
A power inverter in HEVs operates off a DC voltage source, such as a battery, and typically produces a three-phase AC voltage with adjustable frequency and amplitude to control a three-phase AC motor. As an example, FIG. 1 illustrates a block diagram of a conventional electrical motor drive system 10 including a DC power supply 12, a DC bus filter capacitor 14, a three-phase inverter 16, and a three-phase motor/generator 18 for parallel HEVs or battery electric vehicles BEVs. In the motor 18, three-phase currents flowing through the stator windings 20 create a rotating electrical field, causing a rotor to spin.
Power semiconductor devices, such as the insulated-gate-bipolar transistor (IGBT) and diode connected in anti-parallel fashion or the Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET), are often used for the inverter 16 switches, (S1-S6). An electronic controller 22, based on one or more microprocessors, is typically used to control operations of the electrical motor drive system 10. A gate driver circuit 24 is also used to convert the low-voltage digital control signals generated by the controller 22 to higher voltage gating signals that are suitable for turning on or off the semiconductor switches (S1-S6) and to provide galvanic isolation between the power inverter and the electronic controller.
The electric motor 18 is controlled by the inverter 16 and powered by the battery 12 to handle the variations in the driving force demands for optimizing the fuel efficiency of a vehicle. For charging the battery 12, the motor 18 functions as a generator driven by the engine or by the vehicle's inertia during deceleration and produces an AC voltage, which is converted by the inverter 16 to a DC voltage at a level that is suitable to charge the battery 12.
The ratio of conduction to non-conduction time interval for each switch (S1-S6) is determined by using a pulse width modulation (PWM) scheme to adjust the amplitude and frequency of the fundamental component of the inverter 16 output voltage. The PWM switching of the motor currents generates a pulsed inverter DC bus current consisting of a DC component and large ripple AC current components, as shown in FIG. 2 as an example.
Referring now to FIG. 2, the current waveforms at the battery Ibat, the inverter Iinv, the DC bus capacitor ICbus, and the stator windings ia,ib, ic are shown. The root-mean-squared (rms) value of the sum of all the ripple components in the inverter DC-link current can reach up to fifty percent (50%) of the motor current. The inverter DC-bus current can be expressed by equation 1 below.iinv=Idc+Σk=0∞Σn=1∞In,k sin [2π(nfsw±6kfm)t+αn,k]  (1)In equation 1, the first term is the DC component and the second term represents the ripple components, which have frequencies of multiples of the switching frequency (fsw) or their side bands associated with the fundamental frequency (fm).
To absorb the large ripple currents, which are detrimental to the battery 12, and to maintain a near ideal voltage source to the inverter 16, the inverter 16 generally requires the use of a very high performance DC bus capacitor 14. The DC bus capacitor 14 is physically located close to the inverter switches (S1-S6) when installed.
Currently available capacitors 14 that can meet the demanding requirements of HEV application are both costly and bulky. Further, their ripple current handling capability drops rapidly as the ambient temperature increases. Thus, a low temperature liquid cooling system is oftentimes needed to operate the inverters in the engine compartment of an HEV, adding cost, weight and complexity to the vehicle. As a result, conventional inverter designs make it difficult to meet cost, volume and lifetime requirements for HEV applications.